net: qrtr: Implement outgoing flow control

It is possible to flood a remote with messages and cause memory
exhaustion. To avoid this scenario implement the following flow control
handshake. After sending 10 packets to a remote port, set a control
flag on the message and wait for a confirm rx packet from that remote
port before sending more data.

Change-Id: I81c641d4be6d2014df3f0392d140690f6afac2ac
Signed-off-by: Bjorn Andersson <bjorn.andersson@linaro.org>
Git-Commit: 347753ab1cdac06af321950337a54b03f8203960
Git-Repo: https://github.com/andersson/kernel.git


[clew@codeaurora.org: Add commit text and fix checkpatch warnings.
 Use the control packet instead of header addresses for resume tx and
 fix missing mutex init. Change flow to atomic_t to ensure the compiler
 orders the flow control set and signal correctly.]
Signed-off-by: Chris Lew <clew@codeaurora.org>